The present invention is related to methods and apparatus for employing a polyphase machine and inverter circuitry in order to produce a multiphase AC output characteristic that can take on either of two voltage levels.
Generators are traditionally employed to convert rotational power into electrical power, whether the generator is land-based, part of an aerospace system, part of a marine system, etc. Often, it is desirable to be able to configure a generator in order to output more than one voltage level, such as two levels that are different in magnitude by about 2:1. The traditional approach to implementing such a generator is illustrated in FIGS. 1-3.
FIG. 1 is a schematic diagram illustrating a three-phase, dual-winding generator 10. The generator 10 includes a field winding and three sets of phase windings, phase A, phase B, and phase C. Each set of phase windings includes a winding pair, which are in phase with one another and produce about the same voltage as one another as a function of the characteristics of the field winding. Two different AC voltage outputs may be obtained by configuring the sets of phase windings either in parallel or in series.
FIG. 2 is a schematic diagram illustrating the dual-winding generator 10 in a way that produces a voltage V by placing the windings of the respective sets of phase windings in parallel. This configuration produces a polyphase output at nodes A, B, and C including a neutral N. In this configuration, the current from the generator 10 to a load (not shown) is shared among the windings of each phase and, therefore, each winding carries one half of the load current at the full output voltage V. The effective utilization of the generator 10 in this configuration is about 100 percent.
FIG. 3 is a schematic diagram illustrating the dual-winding generator 10 configured in a way to produce an output voltage of 2V. Each of the windings of the respective sets of phase windings is placed in series with one another such that each winding carries the full load current and one half of the output voltage. Again, the effective utilization of the generator 10 in this configuration is about 100 percent.
Although the use of conventional synchronous generators that are configurable to produce different output voltages has been successful to one extent or another, they are difficult to control and do not provide the kind of flexibility that is needed to meet the ever increasing demand for regulated power.
It is desirable to employ permanent magnet alternators (PMAs) to produce a variable voltage, variable frequency output as a function of the alternator speed, rectifying that voltage into a DC source of power, and employing an inverter circuit to convert the DC power into three-phase AC power. This approach offers many advantages, including permitting variable engine speed (resulting in reduced fuel consumption, noise, wet stacking, emissions, size of engine, and in improved engine life). The advantages also include higher PMA efficiency and resultant use of smaller alternator; improved output frequency stability and variability; and improved output voltage stability and transient response. Among the disadvantages of this approach, however, is that the inverter circuit would have to be rated to produce full output current at full output voltage, which is electrically twice the needed deliverable power, even if the inverter does not need to be thermally designed to deliver twice the power.
While it would be desirable to implement the inverter circuit by way of two inverters for connection in series or parallel to produce 2:1 differing output voltages, it is not possible because the configuration of the conventional inverter circuit is limited to a WYE configuration. Although the PMA may be implemented utilizing dual windings that may be placed in series or parallel to produce a DC source of power of 2:1 differing voltage, the conventional inverter circuit cannot achieve this function.
Accordingly, there are needs in the art for methods and apparatus that employ series/parallel configurable inverters in order to convert a DC source of power into a polyphase output of 2:1 differing output voltage.